improve docs (#2068)

* improve decl_storage instance doc

* use decl_event doc accordignly

* automate doc and while allow to extend it on event

* some missed ones

* Update srml/example/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* Update srml/support/procedural/src/storage/transformation.rs

substrate/srml/sudo/src/lib.rs

@@ -58,7 +58,6 @@ decl_module! {
 	}
 }
 
-/// An event in this module.
 decl_event!(
 	pub enum Event<T> where AccountId = <T as system::Trait>::AccountId {
 		/// A sudo just took place.